Add SerialFormatBenchmark for evaluating large format serialization performance

Author: DmitryNekrasov

benchmarks/src/jmh/kotlin/SerialFormatBenchmark.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+@file:Suppress("unused")
+
+package kotlinx.datetime
+
+import kotlinx.datetime.format.char
+import org.openjdk.jmh.annotations.*
+import org.openjdk.jmh.infra.Blackhole
+import java.util.concurrent.TimeUnit
+
+@Warmup(iterations = 10, time = 2)
+@Measurement(iterations = 20, time = 2)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+@Fork(1)
+open class SerialFormatBenchmark {
+
+    @Param("1", "2", "4", "8", "16", "32", "64", "128", "256", "512", "1024")
+    var n = 0
+
+    @Benchmark
+    fun largeSerialFormat(blackhole: Blackhole) {
+        val format = LocalDateTime.Format {
+            repeat(n) {
+                char('^')
+                monthNumber()
+                char('&')
+                day()
+                char('!')
+                hour()
+                char('$')
+                minute()
+                char('#')
+                second()
+                char('@')
+            }
+        }
+        blackhole.consume(format)
+    }
+}